EVERY Island Has Its History, and in the Case of All but Newly-Formed

III. The Origin and Classification of Islands. VERY island has its history, and in the case of all but newly-formed E volcanic islands or coral islets, every island has a double history, that of the island itself and that of its colonisation by the plants and animals which live upon it. The rocks of which an island consists will give us an insight into, though not always a complete knowledge of, its geological history; and a study of its living inhabitants will generally enable us to decide whether it has been colonised as an island or by direct former connection with a continent. Some biologists maintain that the fauna of an island will show whether it has ever been united to a continent or not, and this is the question which I propose to discuss in the present paper, because the answer to it involves some important inferences and conclusions. There are many ways in which an island can be formed; it may be but a portion of a continent severed from the mainland by the erosive action of the sea; it may be the mountainous part of a country which has sunk beneath the ocean; it may have been thrown up from the floor of the ocean by volcanic action, or it may have been built up by the growth of reef-making corals. There are, however, only two ways in which an island can have been populated without the intervention of man; either it must once have been united to a continent and its inhabitants must be the descendants of those that then lived on that continent, or else its tenants must have been transported across the sea by the help of drift-wood, or by birds, or by winds and storms. It is evident, therefore, that in most cases there is likely to be a certain relation between the geological structure of an island and the nature ofits fauna and flora. Islands formed in the ocean, whether by direct upheaval, or by volcanic eruptions, or by coral growths, are not likely to possess a large assemblage of plants or of animals; they may be covered with vegetation, but the animals found on them must be the descendants of occasional waifs and strays. On the other hand, an island which has once been part of a continent will, if it remain large enough, continue to support a large number of animals, and these will generally include a certain number of Mammalia and Amphibia. Islands have consequently been divided into two great classes MARCH, 1893. CLASSIFICATION OF ISLANDS. oceanic and continental-which are defined by Dr. A. R. Wallace in the following terms :- Oceanic islands are" of volcanic or coralline origin, usually far from continents, and always separated from them by very deep sea, entirely without indigenous land mammalia or amphibia, but with a fair number of birds and insects, and usually with some reptiles." " Continental islands are always more varied in their geological formation, containing both ancient and recent stratified rocks. They are rarely very remote from a continent, and they always contain some land mammals and amphibia, as well as representatives of the other classes and orders in considerable variety." 1 As general definitions framed for the purpose of describing the conditions which have governed and limited the geographical distribution of animals, these sentences are doubtless sufficiently accurate, especially as Dr. Wallace admits there are some islands which do not come very clearly under either of these categories; but he proceeds to lay down a canon the truth of which is by no means so apparent. He says : -" The total absence of warm-blooded terres trial animals in an island otherwise well suited to maintain them, is held to prove that such island is no mere fragment of any existing or submerged continent, but one that has been actually produced in mid-ocean. It is true that if a continental island were to be com pletely submerged for a single day, and then again elevated, its higher terrestrial animals would be all destroyed, and if it were situated at a considerable distance from land, it would be reduced to the same zoological condition as an oceanic island; but such a complete sub mergence and re-elevation appears never to have taken place, for there is no single island on the globe which has the physical and geological features of a continental, combined with the zoological features of an oceanic island." Seeing how little we yet know of the geology of distant islands, this is a statement which further knowledge may at any time disprove, and there is, even now, good reason to believe that it is contrary to facts. If this assertion can be proved to be incorrect, I shall claim to reverse Dr. Wallace's argument, and to maintain that inasmuch as an island does exist which combines the geological features of a con tinental island with the zoological features of an oceanic one, then we may assume that the submergence and re-elevation of a continental island can take place, and, consequently, the absence of mammals in an island cannot be held to prove that it has never been united to a continent. Considering the many subsidences and upheavals which are known to have occurred along the borders of continental areas since the beginning of Tertiary time, it would indeed be strange if some tracts, isolated by subsidence, had not been completely submerged for a time, and afterwards raised afresh from the sea. The West Indian 1 " Island Life," by A. R. Wallace, second edition, 1892, p. 243. 190 NATURAL SCIENCE. MARCH, region is one where such an occurrence is very likely to have happened, for deep.water deposits of late Tertiary age occur in many of the islands; while the raised coral-reefs which are found in the same islands. and reach up to a height of 1,800 ft. above the sea, prove that there has been recent upheaval to at least that extent. Many of the smaller islands are volcanic, and may have been thrown up at any time; but Barbados, the most westerly of all the islands, has just the features of which we are in search; in its faunal aspect it is decidedly oceanic, while its geological structure is a curious combination, being partly continental and partly oceanic. The facts of the case are so remarkable that a brief review of them may here be given. Barbados stands on a submarine bank or ridge which slopes away in every direction till a depth of more than 1,000 fathoms is reached. The core and base of the island consists of stratified rocks, ordinary sandstones, days, and limestones, such as are formed in shallow water near a coast-line where rivers of some size carry detritus into the sea, and these strata must have been deposited very near such a shore, for many of the sandstones are composed of large quartz grains, which would not be carried far from land. Above these shallow water strata lie deposits of a totally different character, consolidated oceanic oozes, like those which are now found only in the deeper parts of the ocean, and are known as Globigerina Ooze, Radiolarian Ooze, and Red Clay. All these kinds of ooze occur in Barbados, and there is not only a superficial resemblance between them and the modern oceanic oozes, but a complete identity of structure, and a close analogy in chemical composition; upheaval and exposure to rain and weather have, of course, effected some little alteration, but have not obscured their structure. It is certain, therefore, that the shallow sea and the extensive shore-line which it bordered sank to a very great depth, certainly to more than 1,000 fathoms, and probably to as much as 2,000 fathoms (12,000 feet). The site of Barbados was then part of the ocean-floor, but after a time upheaval took place, and it was gradually raised till it came within the sphere of reef-building corals; a small coral islet was the result, but as the upheaval continued, the earliest reefs were raised above the sea, and the area of the island was gradually enlarged. This process went on till the island attained its present dimensions (about the size of the Isle of Wight). the soft oceanic deposits and the still older sandstones and clays being protected from the erosive action of the waves by a thick coating of coral rock, except over a small area in the north-west part of the island, where the rain has cut deep valleys through the stratified rocks, and by carrying sand and mud into the sea has prevented the growth of continuous coral reefs on that side. Now, an island with such a history must necessarily have received its present fauna and flora in the same casual way as an 1893 CLASSIFICATION OF ISLANDS. oceanic island that had never formed part of a continental area. Accordingly, though Barbados is exceedingly fertile, and though the island when first discovered was clothed with forest and underwood, its native terrestrial fauna is a very small one. There are only two mammals in Barbados which have been supposed to be indigenous, a monkey and a racoon-like animal, but I am informed by Col. Fielden that the monkey proves to be the Green Monkey of Western Africa, Cercopithecus callitrichus, and the " racoon" is a South American animal (Procyon cancrivorus). The monkey was doubtless brought over in slave-ships, and as it is known that the Caribbean Indians frequented the island before it was colonised by Europeans, and as the early settlers had intercourse with the colonists of Guiana, it is quite possible that the Procyon was introduced by man.

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